The unusual arrangement of the surface carbon atoms encountered in graphite nanofibers comprised of graphite sheets (platelets) aligned at an angle not parallel to the longitudinal axis of the nanofiber, offer some unique methods to control the structure of supported catalytic metal particles and induce major changes in their catalytic performance. Graphitic nanofibers comprised of graphite platelets aligned substantially perpendicular to the longitudinal axis are sometimes referred as “platelet” nanofibers herein. Graphitic nanofibers that are aligned at an angle not perpendicular and not parallel are sometimes referred to as “herringbone” nanofibers. It is possible to use such nanofibers as templates for the vapor deposition of metal oxides that are likely to grow in an epitaxial format on the substrate surface. A number of investigations have focused on the modifications in both particle morphology and catalytic performance brought about by supporting metal crystallites on carbon nanofibers (Rodriguez et al. 1994, Hoogenraad et al. 1995, Chambers et al. 1998, Park and Baker 1998). The exposed surfaces of some of these materials consist entirely of graphite edges that are separated by a distance of about 0.335 nm. Such a feature provide a template for the deposition of metal crystallites where the surface atoms adopt arrangements that are not generally encountered on conventional support media, such as active carbon, silica or gamma-alumina.
Experiments performed with nickel particles supported on graphite nanofibers showed that such systems exhibited unusual properties with regard to the selectivity patterns obtained for the hydrogenation of olefins and diolefins when compared to the behavior found when the metal was dispersed on conventional oxide carriers, such as alumina (Rodriguez et al. 1994, Chambers et al. 1998, Park and Baker 1998). This enhancement in both activity and selectivity was attributed to the fact that the nickel crystallites were located on the edge sites of the nanofibers and as a consequence, the arrangement of the metal atoms was governed by the interaction with the carbon atoms in these regions. Under such circumstances, one might expect different crystallographic faces of nickel to be exposed to the reactant gas compared to those present when the metal was dispersed on less ordered materials. This claim was supported by the observations from high-resolution transmission electron microscopy examinations, which revealed the existence of major differences in the morphological characteristics of metal particles supported on graphite nanofibers and gamma-alumina. Close inspection of the metal particles supported on graphite nanofibers showed that they were preferentially aligned with respect to the fiber lattice. Furthermore, the hexagonal-shaped crystallites were very thin and flat, features consistent with the existence of a strong metal-support interaction. It was also suggested that the interaction between the conductive graphite nanofibers and the metal particles was responsible for inducing electronic perturbations in the latter entities that were beneficial with regard to enhancing the catalytic performance of the system.
U.S. Pat. No. 6,485,858 B1 to Baker et al. teaches the use of Pt supported on graphite nanofibers as a catalyst for use in fuel cell electrodes. A 5-wt. % Pt dispersion on the graphite nanofibers was found to give a comparable performance to a 30 wt. % Pt loading on XC-72 Vulcan carbon for the electrochemical oxidation of methanol at 80° C. U.S. Pat. Nos. 5,569,635 and 6,159,892 to Moy et al. teach the use of carbon fibrils (also known as cylindrical multi-walled carbon nanotubes) as catalyst supports. In contrast to “platelet” and “herring-bone” graphite nanofibers, the surface of carbon fibrils (substantially cylindrical nanotubes) consist of graphite basal planes and not edges. When metal or metal oxide particles are dispersed on conventional carbon materials, conventional graphite materials or conventional cylindrical carbon nanotubes, they typically exhibit relatively weak interactions with the basal plane regions of the carbon resulting in the formation of relatively large globular entities (like oil or water droplets). Most of the metal and metal oxide atoms are contained in the globular entities and are consequently unavailable to perform the desired catalytic reaction. It would be highly desirable if the catalytic particles could be deposited in such a manner that they were spread in the form of a thin film over the surface of the carbon. The resulting catalyst-containing structure would give rise to the most efficient use of the catalytic metal or metal oxide and as a consequence, it would be possible to not only optimize the catalytic efficiency of the system, but it would be also possible to reduce the catalyst loading. This condition can be achieved when the metal or metal oxide particles are dispersed on the highly tailored surfaces of “platelet” and “herring-bone” graphite nanofibers.
While significant benefits can be realized by controlling the structural features of metal and metal oxide particles as a result of dispersing such entities on the surface edge sites of “platelet” and “herring-bone” graphite nanofibers, further improvements in catalytic performance are required. In this context, major advances can be achieved if one is not only able to control the morphological characteristics, but also to simultaneously regulate the electronic properties of the supported particles by using a highly tailored conductive support medium. There remains a need in the art for a carbon support material that combines the attributes of the “platelet” and “herring-bone” graphitic nanofibers and which possess high electrical conductivity.